DVB is the European consortium standard for the broadcast transmission of digital terrestrial television. DVB systems transmit a compressed digital audio/video stream, using multi-carrier modulation, such as orthogonal frequency division multiplexing (OFDM). Another popular method of transmitting signals is digital video broadcasting-terrestrial (DVB-T). When broadcasters employ DVB-T, the transmitted signals do not travel via cable. Instead, they move via aerial antennas to a home based receiver.
DVB-T broadcasters transmit data with a compressed digital audio-video stream using a process based on a Moving Picture Expert Group (MPEG)-2 standard. These transmissions can include all kinds of digital broadcasting, including high definition television (HDTV). MPEG-2 signals represent an improvement over the older analog signals, which require separate streams of transmission.
By way of background, in multi-carrier systems, such as OFDM systems, serially-inputted symbol streams are divided into unit blocks. The symbol streams of each unit block are converted into N number of parallel symbols. After the conversion, these symbols, which include data, are multiplexed and added by using a plurality of subcarriers having different frequencies, respectively, according to an Inverse Fast Fourier Transform (IFFT) technique, and are transmitted via the channel in time domain.
In addition to data, these OFDM symbols also include scattered pilot carriers (SPC), continuous pilot carriers (CPC), and reserve tone pilot carriers. These pilot carriers (signals) are used for frame synchronization, frequency synchronization, time synchronization, channel estimation, transmission mode identification, and/or phase noise tracing. The data and the pilot carriers constitute the useful part of the OFDM symbol. As understood by those of skill in the art, these OFDM symbols also include less useful portions, such as a guard interval.
Once the OFDM symbols are captured on a receiver side of the OFDM system, they must be demodulated. OFDM demodulation procedures include, for example, a Fast Fourier Transform (FFT) step, an equalizing and de-interleaving step, and a synchronizing step, among others.
Synchronization of OFDM receivers is performed to locate the useful part of each symbol to which the FFT is to be applied. This synchronization, generally performed in the time domain, can be characterized as coarse synchronization (e.g., initially performed during an acquisition period) and fine synchronization. Fine synchronization improves upon the results achieved during coarse synchronization enough to provide reliable demodulation.
Current techniques for carrier and symbol synchronization during the acquisition period are time-domain based. They also, however, include a significant frequency domain component. That is, although these techniques are primarily time-domain based, portions are performed after application of the FFT. This time domain focus, however, necessitates the use of continuous pilots in order to successfully perform carrier and symbol synchronization.
The time domain component of these traditional techniques does not accommodate the performance of fine frequency offset estimation. Therefore, traditional techniques must perform fine frequency offset estimation in frequency domain. This is achieved by using continuous pilots. It is desirable, however, to perform all aspects of synchronization, including fine frequency offset estimation, in the time domain. Time domain is preferred because it allows for much faster signal acquisition since many more time-consuming steps (such as the estimation of the FFT window) are required before an FFT can be performed.
It is known by those of skill in the art that coarse synchronization can be performed in the time domain. Performing fine synchronization in time domain, however, is not so easily accomplished. Performing fine synchronization in time domain is desirable because of the faster signal acquisition and step reduction advantages noted above. Achieving fine synchronization in the time domain, however, is difficult without the use of the continuous pilots.
One proposed solution for performing fine synchronization in time domain has been to use scattered pilots instead of continuous pilots to perform the phase error correction. That is, pseudorandom sequences are provided to modulate scattered pilots which in turn can be used in a separate process in the time domain. However, for multicarrier systems that use multiple sized FFTs (such as DVB-T2), using pseudorandom sequences in this manner would add extra complexity to the receiver due to the need to receive and process multiple sequences of different sizes.
What is needed, therefore, is an improved pilot sequence structure that can facilitate more efficient receiver synchronization to decrease the complexity of receivers for multi-FFT size specifications. Particularly, what is needed is an improved technique for performing receiver synchronization using a single pilot sequence in time domain.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.